Variable gain amplifiers and rooting amplifiers are amplifiers that use switches. The switches used by these amplifiers are mechanical switches such as reed relays and semiconductor switches (e.g., solid state relays, photoMOS relays, and CMOS analog switches). When compared to semiconductor switches, mechanical relays are large and have a short service life. On the other hand, semiconductor switches are small and have a long service life. However, semiconductor switches pose the following problems when used in variable gain amplifiers or rooting devices. First, the “on” resistance of a semiconductor switch is a source of gain error. Moreover, the nonlinearity of the “on” resistance of a semiconductor switch is a source of harmonic distortion. Furthermore, the capacitance between terminals of a semiconductor switch is a source of “off” isolation reduction at high frequency. In addition, the nonlinearity of the capacitance between terminals of a semiconductor switch is a source of harmonic distortion. It should be noted that the capacitance between the terminals of a semiconductor switch is the capacitance between the terminals of transmission path terminals.
Amplifiers that solve several of these problems have been proposed in recent years (for instance, refer to JP (Kokai) 2001-298,337).
However, the problems attributed to the capacitance between terminals of semiconductor switches have, of course, gone unsolved.
Therefore, the present invention solves the problems of amplifiers with semiconductor switches attributed to the capacitance between terminals of the semiconductor switch. Moreover, the present invention simplifies the adjustment of input impedance of amplifiers with semiconductor switches while solving the problems attributed to the capacitance between electrodes.